A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
United States Patent Application Publication No. 2005/0279946 describes the use of a plasma source to produce EUV radiation for photolithography. It is described that the plasma source may also emit undesirable ions and electrons, which may cause damage to a collector mirror that collects the generated EUV radiation. Various measures are proposed to combat such particles. First of all, a foil trap in combination with a magnetic field is proposed in the path of the EUV radiation between the plasma source and the collector mirror. As an alternative, a grid in combination with a magnetic field is proposed in the path of the EUV from the plasma source to the collector mirror. The grid extends transverse to the direction of propagation of the EUV radiation. The grid serves to slow down ions that travel through the grid, to make their trajectories more sensitive to the magnetic field.
U.S. Pat. No. 6,906,788 describes the use of a first screen positioned in a path of the beam between the radiation system and an optical element. A positive voltage is applied to the first screen to repel positively charged particles away from the optical element. A second screen is positioned in the path of the beam on at least one side of the screen, and a negative voltage is applied to the second screen to repel negative particles away form the first screen.
Foil traps have been found to be effective against relatively slow ions. Unfortunately, foil traps may be less effective against very fast ions (e.g. ions with a kinetic energy corresponding to more than 10 keV).